Silver-containing photothermographic imaging materials (that is, photothermographic photosensitive imaging materials) that are imaged with actinic radiation and then developed using heat and without liquid processing have been known in the art for many years. Such materials are used in a recording process wherein an image is formed by imagewise exposure of the photothermographic material to specific electromagnetic radiation and developed by the use of thermal energy. These materials, also known as “dry silver” materials, generally comprise a support having coated thereon: (a) a photocatalyst (that is, a photosensitive compound such as silver halide) that upon such exposure provides a latent image in exposed grains that are capable of acting as a catalyst for the subsequent formation of a silver image in a development step, (b) a relatively or completely non-photosensitive source of reducible silver ions, (c) a reducing composition (usually including a developer) for the reducible silver ions, and (d) a hydrophilic or hydrophobic binder. The latent image is then developed by application of thermal energy.
In photothermographic materials, exposure of the photographic silver halide to light produces small clusters containing silver atoms (Ag0)n. The imagewise distribution of these clusters, known in the art as a latent image, is generally not visible by ordinary means. Thus, the photosensitive material must be further developed to produce a visible image by the reduction of silver ions that are in catalytic proximity to silver halide grains bearing the silver-containing clusters of the latent image. This produces a black-and-white image. The non-photosensitive silver source is catalytically reduced to form the visible black-and-white negative image while much of the silver halide, generally, remains as silver halide and is not reduced. In most instances, the source of reducible silver ions is an organic silver salt in which silver ions are complexed with organic silver coordinating ligands.
Differences Between Photothermography and Photography
The imaging arts have long recognized that the field of photothermography is clearly distinct from that of photography. Photothermographic materials differ significantly from conventional silver halide photographic materials that require processing with aqueous processing solutions.
In photothermographic imaging materials, a visible image is created by heat as a result of the reaction of a developer incorporated within the material. Heating at 50° C. or more is essential for this dry development. In contrast, conventional photographic imaging materials require processing in aqueous processing baths at more moderate temperatures (from 30° C. to 50° C.) to provide a visible image.
In photothermographic materials, only a small amount of silver halide is used to capture light and a non-photosensitive source of reducible silver ions (for example a silver carboxylate or a silver benzotriazole) is used to generate the visible image using thermal development. Thus, the imaged photosensitive silver halide serves as a catalyst for the physical development process involving the non-photosensitive source of reducible silver ions and the incorporated reducing agent. In contrast, conventional wet-processed, black-and-white photographic materials use only one form of silver (that is, silver halide) that, upon chemical development, is itself at least partially converted into the silver image, or that upon physical development requires addition of an external silver source (or other reducible metal ions that form black images upon reduction to the corresponding metal). Thus, photothermographic materials require an amount of silver halide per unit area that is only a fraction of that used in conventional wet-processed photographic materials.
In photothermographic materials, all of the “chemistry” for imaging is incorporated within the material itself. For example, such materials include a developer (that is, a reducing agent for the reducible silver ions) while conventional photographic materials usually do not. Even in so-called “instant photography,” the developer chemistry is physically separated from the photosensitive silver halide until development is desired. The incorporation of the developer into photothermographic materials can lead to increased formation of various types of“fog” or other undesirable sensitometric side effects. Therefore, much effort has gone into the preparation and manufacture of photothermographic materials to minimize these problems.
Moreover, in photothermographic materials, the unexposed silver halide generally remains intact after development and the material must be stabilized against further imaging and development. In contrast, silver halide is removed from conventional photographic materials after solution development to prevent further imaging (that is in the aqueous fixing step).
Because photothermographic materials require dry thermal processing, they present distinctly different problems and require different materials in manufacture and use, compared to conventional, wet-processed silver halide photographic materials. Additives that have one effect in conventional silver halide photographic materials may behave quite differently when incorporated in photothermographic materials where the chemistry is significantly more complex. The incorporation of such additives as, for example, stabilizers, antifoggants, speed enhancers, supersensitizers, and spectral and chemical sensitizers in conventional photographic materials is not predictive of whether such additives will prove beneficial or detrimental in photothermographic materials. For example, it is not uncommon for a photographic antifoggant useful in conventional photographic materials to cause various types of fog when incorporated into photothermographic materials, or for supersensitizers that are effective in photographic materials to be inactive in photothermographic materials.
These and other distinctions between photothermographic and photographic materials are described in Imaging Processes and Materials (Neblette's Eighth Edition), noted above, Unconventional Imaging Processes, E. Brinckman et al. (Eds.), The Focal Press, London and New York, 1978, pp. 74–75, in Zou et al., J. Imaging Sci. Technol. 1996, 40, pp. 94–103, and in M. R. V. Sahyun, J. Imaging Sci. Technol. 1998, 42, 23.
Problem to be Solved
A challenge in photothermographic materials is the need to improve their stability to light exposure after imaging and processing. Referred to as “Desktop Print Stability” the formation of additional image or “print-out” is usually most evident as an increase in Dmin. This effect tends to be a problem especially under high humidity conditions.
U.S. Pat. No. 3,824,103 (Pierce et al.) describes the use of certain S-carbamoyl compounds as silver salt stabilizer precursors for photothermographic materials at a level of about 0.25 to about 20 moles per mole of photosensitive silver salt.
U.S. Pat. No. 3,506,444 (Haist et al.), U.S. Pat. No. 3,531,285 (Haist et al.) and U.S. Pat. No. 3,301,678 (Humphlett et al.) describe certain sulfur-containing compounds that break down or cleave at elevated temperatures to form a compound that combines with the silver halide in the unexposed and undeveloped areas of the emulsion to form a silver mercaptide that is more stable than the silver halide to light.
Japanese Kokai 61-269147 (Kitaguchi et al.) describes a heat-developable photographic element containing a base or base precursor and a photographic reagent capable of releasing a compound of the formula RSH [where R=alkyl, alkenyl, aryl] on reacting with a base. The requirement of a base or a base precursor adds complexity to the system and does not function well in an acidic photothermographic material.
Japanese Kokai 2002-268180 (Maeda et al.) describes a heat-developable photographic element comprising a compound represented by the formula (1)
where Lv is O, N, or S.
Japanese Kokai 2001-013617 (Nobuaki et al.) describes a photothermographic material comprising a compound represented by formula W1—S—ArH1 where W1 is a variety of blocking groups and ArH1 is an aromatic hydrocarbon group or an heteroaromatic group.
Japanese Kokai 2001-264932 (Kyoko et al.) describes a photothermographic material comprising a compound that is a blocked aromatic or heteroaromatic antifoggant or print-out stabilizer.
There remains a need to find and incorporate compounds into photothermographic materials that improve Desktop Print Stability without sacrificing photospeed and other sensitometric properties.